1. Field of the Invention.
The present invention relates to differentials, and more particularly, to limited slip differentials.
2. Description of the Related Art.
Differentials are well known in the prior art and allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.
The completely open differential, i.e. a differential without clutches or springs, is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel, for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a "spin out" of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior methods of limiting slippage between the side gears and the differential casing use a frictional clutch mechanism, either clutch plates or a frusto-conical structure, and a bias mechanism, usually a spring, to apply an initial preload between the side gears and the differential casing. By using a frictional clutch with an initial preload, provided, for example, by a spring, a minimum amount of torque can always be applied to the wheel having traction, i.e. the wheel located on dry pavement. The initial torque generates gear separating forces which further engage the frictional clutch and develop additional torque. Examples of such limited slip differentials are disclosed in U.S. Pat. Nos. 4,612,825 (Engle), 5,226,861 (Engle) and 5,556,344 (Fox), which are assigned to the assignee of the present invention and are expressly incorporated herein by reference.
The initial preload initiates the development of side gear separating forces which provide further braking action between the side gears and the differential casing. In general, gear separating forces are forces induced on any set of meshing gears by the application of torque to the gears and which forces tend to separate the gears. In a differential, the development of torque will create side gear separating forces which tend to move the side gears away from the pinion gears. When one wheel is on surface having a low coefficient of friction, the initial preload creates some contact and frictional engagement between the differential casing and the clutch mechanism disposed between the side gears and the differential casing to allow the engine to provide torque to the wheel having traction. This initial torque transfer induces gear separating forces on the side gears which tend to separate the side gears to further frictionally engage the clutch mechanism with the casing. The increased frictional engagement of the clutch allows more torque to be developed, thus further increasing the side gear separating forces and limiting the slippage between the side gears and the differential casing.
However, such preloaded clutches are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Additionally, such clutch mechanisms usually employ spring mechanisms which add to the cost and difficulty of manufacture.
Additionally, such a preloaded clutch mechanism may lock the output shafts together in situations where differential rotation is necessary. For example, if the vehicle is making a turn when the wheels are sufficiently engaged on the road surface and a sufficient amount of torque is developed, the differential will tend to lock up the output shafts due to the action of the side gear separating forces created by the developed torque.
Another method of limiting slippage involves engaging a frictional clutch mechanism between the side gears and the differential casing based on the difference in rotational speeds between the two output shafts. The frictional clutch may be actuated by various hydraulic pump mechanism which may be external to the differential case or may be constructed of elements disposed inside the differential casing.
A prior art method of limiting slippage involves using a flyweight governor in combination with a clutch mechanism wherein the governor actuates the clutch mechanism when a predetermined differential rotation rate is detected. However, prior art devices using such arrangements are configured such that the governor almost instantaneously applies extremely high clutch torque to the output shafts, essentially locking the two output shafts together. Applying locking torque in such a manner applies very high stresses on the output shafts and may result in fracturing the output shafts.
Thus, what is needed is a simple, durable and reliable limited. slip differential which can effectively provide torque to the wheel with traction.
What is also needed is a limited slip differential which is responsive to speed difference to provide the limited slip function only when required, i.e. limited slip when one wheel has lost traction, but relatively open when sufficient torque is developed.
What is also needed is a limited slip differential which applies a predetermined amount of clutch torque in response to a loss of traction.
Lastly, what is also needed is a limited slip differential which applies only a predetermined amount of clutch torque during a loss of traction.